A millstone has conventionally been used as means for grating tea leaves, grains, and other objects to be grated. The millstone is placed with a fixed lower mill and a rotating upper mill facing and being in contact with each other. An object guided to a gap between a grinding surface of the lower mill and a grinding surface of the upper mill from a portion around a center of rotation in an upper portion of the upper mill is pulverized as being grated by a flat portion and grooves provided in the grinding surfaces.
While the stonemill has traditionally been fabricated by working stones, some apparatuses in which such a traditional millstone is turned by electrically powered drive are available for a method of mass production of powders. On the other hand, a manual or electrically powered milling machine of a millstone type having a function comparable to the traditional millstone has been developed.
Japanese Patent Laying-Open No. 2005-199242 (PTD 1) discloses an electrically powered milling machine of a millstone type. This electrically powered milling machine has a shape of a mill like a traditional millstone (see FIG. 6 of the document).
Namely, in a surface (grinding surface) of a mill having a cylindrical outer shape, a plurality of grooves are formed in parallel at a prescribed pitch in each area resulting from division into eight parts around the center of rotation and a flat portion is provided in a portion other than the grooves. A portion of interest is grated by shear at an edge of the groove and by an effect of grinding at the flat portion as a result of abutment between an upper mill and a lower mill at the grinding surfaces and rotation of the upper mill and the lower mill relative to each other. Furthermore, PTD 1 discloses capability to suitably grate an object to be grated by setting a proportion of groove portions in opposing surfaces to be at least approximately twice as high as a proportion of the flat portion.
Japanese Patent Laying-Open No. 2001-275843 (PTD 2) discloses a tea dispenser which dispenses powder tea grated by an electrically powered milling machine of a millstone type. Here, a technique to provide a shutter preventing steam from entering a powder tea supply side and a technique to agitate powder tea in a storage portion are disclosed. A traditional shape of a mill is disclosed for grating of tea leaves as in PTD 1.